This invention relates to backflow preventer devices for fluid flow lines, and more particularly to improvements in relief valves of such devices.
Backflow preventer devices are conventionally installed in residential water distribution systems to prevent backflow of waste water. The backflow preventer devices are designated to discharge backflow to the atmosphere in order to prevent polluting the upstream water supply. Such backflow preventer devices typically include an upstream check valve and a downstream check valve. Between the two check valves an intermediate or reduced pressure zone provides for drainage to the atmosphere through a relief valve if the second check valve fails to seal when the upstream supply pressure is suddenly reduced or the main flow is entirely is shut off. U.S. Pat. Nos. 4,276,897, 3,171,423, RE 21,323 and 2,174,743 describe backflow preventer devices and relief valves. Serious problems experienced with many relief valves are the tendency for such valves to freeze closed and excessive friction in the relief action, particulary in water distribution systems. These disadvantages are eliminated in the backflow preventer device according to this invention.